It's a sac which is formed from the epidermis and which has a hair and a sebaceous gland. The sebaceous gland produces sebum which lubricates the hair and makes up a part of the oily phase of the hydrolipidic film.

e have about 400 to 900 sebaceous glands on every 2cm of our face, or 5 to 10 times that of the body. The hormones regulate sebum secretion. Under hormonal pressure (androgens), the sebum can be produced in excess.

If the canal is obstructed, the sebum and dead skin cells are trapped. If the pore remains opened it becomes a blackhead. If the pore closes, there can be a bacterial proliferation provoking inflammation, which becomes a spot.

A long-term excess of sebum causes oily skin: large, exposed pores, thick cutaneous folds. This is tough skin.
Adolescents can be affected by acne because puberty causes strong hormonal changes.

Adult skin tends to be oily, and women during menstruation, pregnancy, and menopause can also be affected.
The occasional spot doesn't discriminate and can affect everyone.